(1) Field of the Invention
The present invention relates to a radiocommunication apparatus and a radiocommunication system employing a direct spread CDMA (Code Division Multiple Access) communication scheme.
(2) Description of the Related Art
In recent years, a code division multiple access scheme employing spread spectrum communication--which is a multiple access scheme having a superior utilization efficiency of frequencies--has received attention as a transmission scheme for use in radiocommunication. This CDMA scheme is a leading candidate for the next-generation mobile communication system employed for radiocommunication between the base station and a plurality of mobile stations.
More specifically, the CDMA communication scheme is one in which multiple access is effected by spreading radio waves from each station through use of a high speed code inherent to the station. A transmitting-station transmits signals, while changing a spread code for each communication channel, at the same time over the same frequency band, and a receiving station reconstitutes only the information signal received over a communication channel assigned to the receiving station through use of its own spread code. A direct spread CDMA communication scheme is commonly employed, wherein the spectrum spreading is effected by modulating a carrier wave through use of a series of codes for spreading.
In a downward communication (communication from the base station to the mobile stations) of the direct spread CDMA communication scheme, signals are spread through use of different spread codes, and the thus-spread signals are transmitted to the mobile stations from the base station. The mobile station generally carries out despread processing for the received signal through integrate-and-damp process (or by correlation processing) of a code assigned to the mobile station (a spread code) and the received signal, whereby it obtains a desired wave.
FIG. 13 illustrates a radiocommunication system to which a common direct spread CDMA communication scheme is applied. A radiocommunication system 10 illustrated in FIG. 13 is comprised of a transmitter 7 connected to an antenna 7A, a receiver 8 connected to an antenna 8A, and a radio circuit 9 through which the transmitter 7 and the receiver 8 are connected together.
The transmitter 7 is comprised of a serial/parallel converter (S/P) 70, mixer circuits 71a and 71b, a control section 72, a spread code generator 73, transmission filters (TX filters) 74a and 74b, digital/analog converters (D/A) 75a and 75b, mixer circuits 76a and 76b, a voltage-controlled oscillator (a local oscillator) 77, a 90-degree phase shifter 78, an adder 79, and an amplifier (a low-noise amplifier) 17.
The mixer circuits 71a and 71b, the control section 72, and the spread code generator 73 function as a spectrum spreading section 52, whereas the mixer circuits 76a and 76b, the voltage-controlled oscillator 77, and the 90-degree phase shifter 78 function as a frequency converter (an up-converter: U/C) 53.
The receiver 8 is comprised of a hybrid device (H) 80, mixer circuits 81a and 81b, a voltage-controlled oscillator (a local oscillator) 82, a 90-degree phase shifter 83, receiving filters (RX filters) 84a and 84b, analog/digital converters (A/D) 85a and 85b, correlators 86a and 86b, a code generator 87, and a control section 88.
The mixer circuits 81a and 81b, the voltage-controlled oscillator 82, and the 90-degree phase shifter 83 function as a frequency converter (a down-converter; D/C) 62. Further, the correlators 86a and 86b, the code generator 87, and the control section 88 function as a spread demodulation section 89.
In the radiocommunication system 10 illustrated in FIG. 13 that has the previously-described circuit configuration, when a signal is transmitted from the transmitter 7 to the receiver 8, the serial/parallel converter 70 converts information data (serial data) into parallel data and divides the thus-converted parallel data into two signals. The mixer circuits 71a and 71b carry out spectrum spread processing for the parallel data. That is, the two thus-converted parallel data signals are subjected to spread spectrum processing through use of a signal which is generated by the control section 72 according to a spread code received from the spread code generator 73.
The signals having their spectra spread are then limited to a desired band by the transmission filters 74a and 74b. The signals are then converted into analog signals by the digital/analog converters 75a and 75b, and the frequencies of the thus-converted analog signals are converted by the frequency converter 53. More specifically, the frequency of a signal output from the digital/analog converter 75a is converted by the mixer circuit 76a through use of a signal (i.e., a local signal) which has a frequency corresponding to a signal received from the voltage-controlled oscillator 77. On the other hand, the frequency of a signal output from the digital/analog converter 75b is converted by the mixer circuit 76b through use of a signal from the 90-degree phase shifter 78, which shifts the phase of the signal from the voltage-controlled oscillator 77 through an angle of 90 degrees. These two signals having their frequencies converted are added by the adder 79, and a resultant signal is amplified by the amplifier 17. Then, the thus-amplified signal is transmitted from the antenna 7A in the form of radio waves.
In contrast, in the receiver 8, the signal received by the antenna 8A is divided into two signals by the hybrid device 80. The frequencies of the signals are converted by the frequency converter 62. At this time, the frequency of one of the two signals is converted by the mixer circuit 81a through use of a signal received from the voltage-controlled oscillator 82. The frequency of the other signal is converted through use of a signal from the 90-degree phase shifter 83, which shifts the phase of the signal from the voltage-controlled oscillator 82 through an angle of 90 degrees.
Subsequently, these signals are limited to a desired band by the receiving filters 84a and 84b, and they are converted into digital signals by the analog/digital converters 85a and 85b. The thus-digitally converted signals are subjected to despread processing performed by the spread demodulation section 89.
More specifically, the control section 88 of the spread demodulation section 89 performs determination of code timing and code selection through use of a spread code of the receiver itself generated by the code generator 87. The correlators 86a and 86b carry out despread processing based on the output from the control section 88 so as to output the despread information.
In the previously-described radiocommunication system 10, the transmitter 7 and the receiver 8 can be used; for example, as the base station (a master station) and the mobile station, respectively. Specifically, audio or data information is transmitted between one master station and a plurality of mobile stations. For example, as illustrated in FIG. 14, if information is transmitted to three mobile stations (MS) 8a, 8b, and 8c from the base station (BS) 7a, because signals can be multiplexed through use of codes according to the CDMA scheme, signals can be sent to all the mobile stations, 8a, 8b, and 8c, at the same time over the same frequency.
Further, the signals to be sent to the mobile stations 8a, 8b, and 8c are respectively comprised of an information portion (see arrow D in FIG. 14) and a common signal portion (see arrow E in FIG. 14). Contents which change according to the respective mobile station, 8a, 8b, or 8c (information to be sent to the mobile station 8a, information to be sent to the mobile station 8b, and information to be sent to the mobile station 8c) are written into the respective information portions.
In contrast, information common to the mobile stations 8a, 8b, and 8c are written into the respective common signal portions (i.e., hatched portions). An example of such a common signal is a pilot signal that is used in estimating a fading propagation path inherent to mobile communication. Each of the mobile stations 8a, 8b, and 8c carries out despread processing for a received signal by integrate-and-damp process of the code assigned to each mobile station and the received signal (the information portion and the common signal portion).
However, in the radiocommunication system 10 applicable to the previously-described CDMA scheme, the common signal portion of the signals sent to each of the mobile stations is despread by the mobile station through use of the spread code therefor. Therefore, each of the mobile stations cannot utilize a signal power of common signal portions sent to other mobile stations, inturn making it impossible to improve the signal-to-noise ratio (S/N) of the received signal.